Conventional and commercially available thermoplastic polyurethanes and polyureas comprise soft/rubbery polymer segments covalently linked to incompatible hard/crystalline that provide physical crosslinks and reinforcement. The ultimate properties of these physically crosslinked networks are primarily due to the nature, overall composition, and morphology of the soft and hard segments. The nature and extent of hydrogen bonding within the hard segments, and between the hard and soft segments, largely determines mechanical properties and proccessability. Thus, polyureas, whose mechanical properties are, as a rule, superior to polyurethanes, have heretofore never been thermally processed (i.e., melt processed) because they contain a variety of strong H-bonds. These bidentate H-bonds between urea groups (e.g., —NHCONH—) degrade before flow even upon moderate heating, say to about 240° C. Hence, conventional polyureas do not melt and will degrade before melting.
As a consequence, all known, conventional polyureas are processed in solution (e.g., by dry spinning) by the use of environmentally unfriendly solvents. One well known example of a polyurea fiber processable only by solution techniques (e.g., dry spinning) using a strongly H-accepting solvent (e.g., dimethylformamide), is the spandex or elastane polyurea fiber available from E.I. du Pont & Nemours & Co. under the trade name Lycra®. Because of the use of solvents such as dimethylformamide, solution proccessability of polyureas is costly, cumbersome, and environmentally unfriendly.
Accordingly, a need exists for thermoplastic elastomers, such as polyureas and polyurea-urethanes, that can maintain their desired excellent mechanical properties, but are not so costly, cumbersome and environmentally unfriendly as those polyureas produced in solution.
Heretofore, attempts have been made to render polyureas melt processable. However, all prior processes undertaken to attain thermal proccessability have called for major changes in the synthesis of the polyurea. For example, one recognized method to attain melt proccessability of polyurea is to eliminate the use of chain extenders (CEs) and drastically reduce the hard segment content (to less than 14%). Reducing the number of chain extenders in the polyurea will reduce the ability of hydrogen to bond to something (e.g., a nucleophilic group) that would prevent the melt processing of the urea. Similarly, at least one patent application (US Patent Application Publication No. 2009/0036598) prepares polyureas by the reaction of polyisocyanates and polyamines with very low hard segment content. However, the lowering of the hard segment content also reduces significantly the mechanical properties of the polyureas, leaving undesirable characteristics.
Even more recently, others have reported melt processable polyureas with slightly higher amounts of hard segments (15-25%) by using branched CEs. Again, however, the branched CEs reduce many, if not all, of the desired mechanical properties, including tensile strength, elongation, Shore A hardness, etc.
Thus, the need exists for a melt processable thermoplastic elastomer having at least 30% hard segment content and exhibiting good mechanical properties.